When I have an isotropic point-like source that radiates isotropically in 3-D space, how can I calculate the number of events that reach a screen a certain distance away, with the probability as a function of (transversal) x and/or y on that screen.Hence, I want to know how to derive a function for the amount of events that reach a certain region on a flat screen, when the origin of these events is some isotropic point-like source. Any help would very much be appreciated. Best regards,